The present invention relates to a roller. More particularly, the invention relates to a roller compensated and/or adjusted with respect to its deflection.
The roller is a press roller or calender roller in a paper machine. A load is imposed at the ends of the roller. The roller has a non-rotating axle, at the ends of which the load is imposed, and a cylindrical shell supported by bearings on the axle. The roller axle is made to form a magnetic core, in which devices which produce the magnetomotive force are arranged in such a way that the magnetic flux through the air gap between the pole shoes of the magnetic core and the roller shell brings about a compensating force field between said shell and said magnetic core.
The roller of the invention is intended to be used in the pressure treatment of web-like sheet material, when defining a nip in association with a mating roller.
It is generally well known that rollers of the aforedescribed type, particularly when loaded by the imposition of a load at their ends, suffer deflection, so that the linear pressure at the press nip is greater at the region of the outer ends of the nip than at the central region of the nip. Attempts have been made to avoid this drawback by providing the rollers with a convex exterior surface. When using conventional convex rollers, however, it is possible to obtain, for example, a uniform linear pressure only with a given load. In order to avoid this drawback, it has been proposed to provide for controllable convex rollers. An example of a controllable convex roller is the known Kusters roller (U.S. Pat. No. 2,908,964), wherein a pressure fluid chamber is situated between the stationary roller axle and the outer rotating roller shell at a selected sector thereof. It is possible to compensate for the deflection of the roller shell by controlling the pressure in the pressure fluid chamber. However, the Kusters roller suffers from the drawback of having sealing difficulties accompanied by unavoidable leakage of pressure fluid. Another drawback of the Kusters roller is that it responds relatively slowly to changes of pressure; an interval of approximately 30 seconds being required for an increase in pressure while an interval of approximately 10 seconds is required for a decrease in pressure to be effected. The consequence of these drawbacks is that when there is a change in pressure a considerable quantity of paper is wasted when such a roller is used in paper machines.
There are known deflection-compensated rollers such as the so-called CC roller of the Beloit Co. In the CC roller, pressure shoes frictionally rub against the inner surface of the roller shell and a load is imposed on these shoes via a pressurized fluid. In addition, it is relatively difficult to provide controls capable of achieving asymmetrical effects axially on the roller. Compensating forces are provided via pressure fluid chambers or friction shoes which make high demands on the smoothness of the inner surface of the press shell. This adds to the manufacturing costs of the CC roller.
Press rollers which use electromagnetic forces are also known and the objective of the present invention is to further develop such rollers. In this connection, reference may be made to U.S. Pat. No. 3,456,582. However, the roller disclosed in this patent, in addition to the fact that it differs from the present invention in its details, is also different in principle in that in said U.S. patent the roller is not loaded at its ends.
DT-OS No. 1,761,641 discloses a deflection-compensated roller utilizing permanent magnets. The roller includes permanent magnets in association with both the fixed axle and the rotating shell. The compensating forces are adjusted by axially adjusting the mutual position of the roller axle and the shell. This changes the position of the roller shell with respect to the permanent magnets in the axle with the result that the compensating forces are simultaneously adjusted.
In applicant's Finnish Pat. No. 52,394 (related U.S. Pat. No. 4,062,097), deflection-compensation of a magnetic roller is achieved by creating a magnetomotive force via devices arranged in association with the two essentially coaxial cylindrical parts. Another device is provided in association with the roller axle for producing a magnetomotive force between the opposite sectors of the roller axle. The combined action of the magnetic fields produced by the magnetomotive force attracts the axle and shell to each other on one side and repels the axle and shell from each other on the opposite side.
The present invention is most clearly similar to Finnish patent application No. 2283/67, corresponding to U.S. Pat. No. 3,489,079. In the roller described in this publication, there is a magnetic shoe device of V-shape which creates an attractive force at the two air gaps between the two poles of the magnetic shoe device and the inner shell of the roller. In this device, the angle between the magnetic shoes is relatively small, so that the effective air gap is relatively small and a considerable part of the inner shell of the roller is inefficiently used.